Electrical casing - Experimental investigation of electric field on casing to improve cement-casing bonding
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Cementing is an essential part of every drilling operation. Protection of the wellbore from formation fluid invasion is one of the primary tasks of a cement job. Failure in this task results in catastrophic events, such as blowouts. Therefore, in order to save the well and avoid risky and operationally difficult remedial cementing, slurry must be optimized to be resistant against gas migration phenomenon. In this paper, the principal aim is to investigate the application of electric field on casing in order to improve the cement placement and quality of cement-casing bonding. The cement particles are electrically charged, so it is possible to manipulate the composition and the structure of the interfacial zone existing between a stainless steel pipe and cement. Previously, in experiments done at SINTEF Petroleum Research, it was found that bonding was significantly improved at the positive electrode. The idea is that the negatively charged particles are attracted to the positive electrode. This project is based on that idea. Mainly laboratory and experimental work is done in order to continue the investigation of the application of electric field in cement-casing interface.The laboratory and experimental work include the design of a set-up where it is possible to pour a cement slurry and immerse two stainless steel pipes and apply voltage between them. One of the consequences of the application of electric field to cement slurry is the appearance of electro-kinetic processes such as electrophoresis, in which the negatively charged particles are accumulated in the positive electrode. After the application of the electric field a pipe extraction test is done in order to analyze the properties of the cement and study the effect. By means of this test, the shear bond strength is measured and the maximum force value at which the pipe is separated from the cement is obtained. Many samples have been made changing variables such as the voltage value, the curing time or the exposure time of electric field. After the experimental process, all the results obtained have been analyzed and compared, reaching the conclusion that the force measured in the positively charged pipe is greater than the force measured in the negatively charged pipe. This conclusion is fulfilled as long as the voltage between the two electrodes is less than voltage needed by the water to produce the hydrolysis. Finally, some recommendations for further work are proposed. This project has been carried out under standard conditions of temperature and pressure (25ºC, 1 atm), so it is considered relevant to perform the lab experiment at higher temperature and pressure, i.e. 66ºC and 20 bar. It is also proposed to continue the research by applying the electric field between a rock formation and the casing, to see the behavior of the cement particles.